Which cube is a unit cube?

A.a cube that is 3 yards long, 3 yards wide, and 3 yards high

B.a cube that is 2 feet long, 2 feet tall, and 2 feet high

C.a cube that is 5 meters long, 5 meters wide, and 5 meters high

D.a cube that is 1 inch long, 1 inch wide, and 1 inch high